Low-lying neutronfp-shell intruder states in27Ne

Abstract

The quenching of the N =20 shell gap in neutron-rich nuclei is investigated by studying the singleparticle structure of 27 Ne via neutron transfer using a 26 Ne beam. Two low-lying negative parity intruder states have been observed, the lowest of which is identified as J π = 3/2 − , confirming earlier speculations. A level identified as 7/2 − is observed higher in energy than the 3/2 − , contrary to the ordering at β-stability and at an energy significantly different to the predictions of previous shellmodel calculations. The measured energies and deduced spectroscopic factors are well reproduced in full (0,1)-ω 0s-0p-0d-1s-0f -1p calculations in which there is a significant ad-hoc reduction (∼ 0.7 MeV) in the N =20 shell gap. Neutron-rich nuclei often exhibit structural behaviour significantly different to stable nuclei, with a striking example being the "island of inversion" in the A ≃ 32 region of neutron rich nuclei [1,2]. The nuclei in this "island" are deformed rather than spherical owing to residual interactions and quenching of the N =20 magic number through the migration in energy of the shell-model orbitals [3]. This migration is known to be due, in part, to nucleon-nucleon tensor forces and to three-body N N N forces [4,5]. In adjacent more weakly bound nuclei, the migration may also be affected by the proximity of the continuum [6].The N =20 shell gap seen in nuclei near stability arises from the separation of the 0d 3/2 orbital and the negative parity orbitals (0f 7/2 , 1p 3/2 ,.. [10]. Guided by shell model expectations, the 885 keV level was tentatively identified as the lowest 1/2 + state and the 765 keV level was inferred to have a negative parity of 1/2 − , 3/2 − or 5/2 − . The 1/2 + is weak since it is populated mainly via pair-excited components in the 26 Ne ground state, in single-step transfer. In single-neutron knockout at higher energies, both levels were seen and the angular momentum of the removed nucleon was assigned as ℓ=0 or 1 [11], consistent with the results from the (d,p) reaction study [10]. These two excited states were also observed in the p( 28 Ne, 27 Neγ) reaction at intermediate energy [12] but no further information concerning spins was obtained [37]. No evidence for a 7/2 − state has been reported, despite a clear prediction that it should exist at an energy close to the 3/2 − state [11,15]. The most direct means to probe the evolution of orbital energies is to measure the strength of single-particle states where a neutron is transferred into orbitals that are otherwise empty. The present experiment was designed along these lines to populate the 7/2 − and 3/2 − 0f -1p shell states and any other strong single-particle levels via (d,p) transfer and to determine their properties.A beam of 26 Ne ions (∼100% pure) at 9.8A MeV was provided by the SPIRAL facility at GANIL and

Spectroscopy ofNa28: Shell evolution toward the drip line

“…Recent experimental observations [21][22][23][24] have brought remarkable credit to the theoretical description of Fig.1 around (N=16, Z=8) suggesting an inversion between the 1p 3/2 and 0f 7/2 orbits in the N=15 and N=17 Ne isotopes. These experiments have also demonstrated that the N=20 gap is small.…”

One-neutron removal fromNe29: Defining the lower limits of the island of inversion

“…4 were observed in a recent in-beam γ -ray spectroscopic study [22], although spin-parity assignments were not made. In the case of the excited states of 27 Ne, the SDPF-M interaction has some difficulty predicting the energy of the 7/2 − level, found near 2 MeV in a recent 26 Ne(d, p) reaction study [21]. 27 Ne is based on Refs.…”

“…Recent 26 Ne(d, p) reaction studies have demonstrated that the lowest 3/2 − state is at 765 keV, while the 27 Ne ground state is 3/2 + [18][19][20][21] (see also Fig. 4 in Sec.…”

“…In addition to 31 Ne, there is now a body of experimental data on the very neutron-rich Ne isotopes [12,[17][18][19][20][21][22][23]. The structures of these isotopes evolve considerably as one moves toward the neutron drip line and are complex.…”

Enhanced E1 transition between weakly-bound excited states in the nucleus Ne27

“…(The spectroscopic factor with rather large experimental errors obtained from a d( 26 Ne, 27 Ne)p reaction can be found, for example, in Ref. [6].) In contrast, I take the model that both the first (3/2 − ) and second (1/2 + ) excited states as the deformed 26 Ne core plus one neutron in the quadrupole-deformed potential.…”